For this 52-year-old Briton, diving is about exploring, learning and understanding and respecting the natural world.

He said it can be a bit unnerving coming across a strange creature or large fish under the sea.

"As a diver, when you see something special under the water, you become curious immediately and then you maybe feel fear and respect. I like to convert fears into understanding and respect. I teach my students to relax through understanding."

Some animals get a bad rap, Stein said. He urges all his students to have open minds while diving.

"Strange animals and sharks aren't like they are in the movies. I saw a Hammerhead Shark when I was diving in Borneo last year. I was at the depth of 50 meters and we were five meters from each other. The shark looked right at me and I was thrilled."

Same same but different

The sea is always changing and different, Stein says, and no two dives are ever the same.

"Every time I dive have new experiences and I see things differently... the underwater world is same same but different every where. It is lovely to discover the small differences whenever I go somewhere and look for something interesting to share with other divers."

Stein's journey of discovery has really just begun as he's now surveying as much underwater terrain as possible to make full-fledged dive maps of popular areas around Phu Quoc, Con Dao, Hoi An, Nha Trang, and Whale Island.

He teaches his students to identify dozens of species of sea-dweller and he also instructs them in underwater photography.

Beginners can even practice with Rainbow at swimming pools in HCMC and Hanoi before going out to the ocean for a real discovery.

Stein says he has no intention to ever stop pathfinding or water-trailblazing. He never tires of life under the sea.

"The coral reefs in different colors, different kinds of fishes with different kinds of behaviors all gathering at the rocks, making friends, dancing."

His favorite thing to do is go out diving without a plan.

"Getting lost is a part of the excitement and often brings the greatest experiences."

A teacher's life

The journey from money-man to professional diver was a long time coming for Stein, who first began diving in Thailand and the Maldives when he was 12 years old. Even as a busy London banker in his 30s, Stein managed to find time to become a certified diving instructor, teaching people in his free time. But it was only when he arrived in Nha Trang that his real dream came true. To Stein, opening his own dive center was the perfect way to combine his two loves: finance and diving.

Now he's cooperating with schools in Vietnam like the Australia International School and the British International School to share what he's learned with young people and teach them about protecting the environment. "I love teaching children as they are fearless and if they like it they will do it. They genuinely love the experience and they show emotion."